- Case Sun Products Company An Exercise 44 In Variables Control Charting Case Date March 1996 Alan Harrison Sun Products 1 (66.85 KiB) Viewed 72 times
- Case Sun Products Company An Exercise 44 In Variables Control Charting Case Date March 1996 Alan Harrison Sun Products 2 (43.49 KiB) Viewed 72 times
- Case Sun Products Company An Exercise 44 In Variables Control Charting Case Date March 1996 Alan Harrison Sun Products 3 (40.3 KiB) Viewed 72 times
- Case Sun Products Company An Exercise 44 In Variables Control Charting Case Date March 1996 Alan Harrison Sun Products 4 (17.57 KiB) Viewed 72 times
CASE Sun Products Company: An exercise 44 in variables control charting Case date March 1996 Alan Harrison Sun Products Company Sun Products Company (SPC) makes a range of components for various customers in the automotive industry at home and abroad. One of these customers made an approach a year or so ago with a quality concern about the sunroof which was sup- plied by SPC. The customer delegation told the Sun QA Manager, Fred Jarvis This is on our top 40 list of quality concerns at present. The height of the glass on the roof is often noticeably uneven, or sits too proud of the car body. We'd like to take a look around your processes to see if there are any immediate improvements we can make After looking round the sunroof line at SPC, the customer delegation suggested to Fred that some improvements should be made to materials handling within the Sun works, and in the way that parts were transported from Sun suppliers. A detailed inspection of the production process with Fred and his colleagues had been made, and a process flow chart drawn up. But attention had eventually focused on the profile set- ting operation where the glass was fitted into the sunroof mechanism. A sketch of this is shown in Figure 44.1. A 100 per cent inspection of the profile was made by probes at the end of the production process. The probes measured the height of the glass at four points on the sunroof near to the corners, represented by P1, 12, P3 and 14. The customer specification called for a profile measure with a tolerance of +1.00mm for each point to N. If the probes detected a condition which was out of specification for one of these points, the inspector called for the profile setting to be altered A month later, the customer delegation returned to Fred, and told him that they had made further investigations of their own assembly proces We have made a number of immediate improvements on our assembly line, such as giving the operator a step block to make the fitting process more exact and less visual. Now we want you to introduce variable control charts on your profile setting operation You are generating a lot of data at the inspection stage, but you're not doing enough with it. Further, you're reacting to events rather than controlling them." These comments had come as rather a shock to Fred, who thought that Sun had installed the best kit, and that it was virtually impossible (given 100 per cent inspec- tion of the profile for defective products to be shipped. Nevertheless, Fred got to 414 Part 6. Quality planning and control
Figure 44.1 The sunroof at Sun Products Company Motor Glass NB: Sketch only: not to scale) work, first tackling a process capability study. This came as an even greater shock! The process was clearly out of control. It seemed very difficult to maintain specifica- tion on all four points at the same time. Thus C values of less than I were initially quite common. Fred concentrated first on getting rid of special causes of variability For example, he found that when the glass supplier changed batches, the curvature of the glass could change significantly and cause problems at Sun. But once these problems had been recognised and dealt with, Fred was satisfied that control chart- ing could start. Although the inspection process would be left at 100 per cent for the time being, Fred decided on taking two samples each hour and recording the difference to spec. ification for all four points P1 to 14. Samples would be taken for the two readings made closest to the hour. Tables 44.1 and 44.2 show the data which were used for calculating the grand average, and the upper and lower control limits for the aver- ages chart (X Bar Chart) for point 3. They also provide the data used to calculate the upper controllimit for the range chart ( Chart) for 13. Sun Products Company. An exercise in variables control charting 415
-0.1 -0.3 -0.1 Table 44.1 P3 data used to calculate the process capability indices Reading 1 Reading 2 Saturday 2/12 06.00hrs +0.6mm +0.2mm 07.00 +0.3 +0.2 08.00 +0.4 +0.1 09.00 +0.1 -0.3 10,00 -0.2 +0.0 11.00 -0.0 +0.0 Monday 4/12 06.00 +0.2 +0.2 07.00 -0.3 +0.1 08.00 -0.3 +0.0 09.00 +0.1 +0.3 10.00 +0.6 +0.4 11.00 +0,0 -0.2 12.00 +0.5 +0.0 13.00 +0.1 +0.2 14.00 +0.1 +0.0 15.00 +0.2 +0.3 16.00 +0.4 +0.3 17.00 +0.1 -0.1 Tuesday 5/12 06.00 +0.0 +0.6 07.00 +0.2 +0.0 08.00 -0.3 -0.1 09.00 +0.0 -0.2 10.00 +0.5 +0.2 11.00 +0.1 -0.2 12,00 -0.2 -0.1 13.00 +0.0 +0.1 14.00 +0.1 15.00 +0.0 +0.4 16.00 -0.3 -0.1 Table 44.2 P3 data from a subsequent day's production Reading 1 Reading 2 Thursday 18/1 06.00 -0.3 -0.1 07.00 -0,4 08.00 -0.2 -0.3 09.00 -0.2 -0.2 10.00 -0.5 -0.2 11.00 -0.4 -0.8 12.00 -0.7 13.00 +0.1 14.00 -0.1 -0.3 15.00 -0.3 -0.1 16.00 +0.1 +0.2 17.00 +0.1 -0.1 18.00 -0.2 -0.1 19.00 -0.1 +0.2 Friday 1911 06.00 -0,6 -0.4 07.00 +0.0 -0.4 08.00 -0,3 -0,8 09.00 -0.5 -0.2 10.00 -0.5 -0.2 11.00 -0.5 -0.2 12.00 -0.0 +0.0 13.00 +0.1 -0.2 14.00 +0.2 15.00 -0.0 +0.6 16.00 +0.2 -0.1 17.00 +0.2 -0.1 18.00 +0,0 +0.0 -0.1 +0.1
2 From Table 44.1, calculate the grand average and the upper and lower control limits for the P3 profile height setting process at Sun. Use the formulae and con- stants given on the control chart, Plot the hourly data on the chart. Given that action on special causes is called for if there is any point outside the control limits, or if there is a run of seven points all above or all below the central line, is the process under statistical control? Does this mean that Sun no longer ships defective product? 416 Part 6. Quality planning and control